Generally, methods of ring-opening polymerization use metal compounds, which are likely to pose problems of contamination or of compatibility in contact with foods or with living tissues. Other methods use acids, which are corrosive in the manufacturing processes.
The (co)polymers of lactones such as s-caprolactone are polymers that are of industrial interest in various fields, notably on account of their biocompatibility, their physicochemical properties and their good thermal stability up to temperatures of at least 200-250° C.
A method for preparing these copolymers was notably described by Jérome et al. in Macromol, 2002, 35, 1190-1195. It consists of copolymerizing δ-valerolactone with a macroinitiator, either poly(ethylene glycol) or monomethoxypoly(ethylene glycol), in the presence of ethereal hydrochloric acid (HCl.Et2O) in dichloromethane at 0° C. This method uses a monomer concentration of 3 mol·l−1, 3 equivalents of acid relative to the hydroxyl functions of the macroinitiator, and the diblock and triblock polymers obtained after 2-3 h have maximum number-average molecular weights Mn from 9500 to 19,000 g/mol, with a polydispersity index from 1.07 to 1.09.
This method requires the use of a relatively large amount of acid, which is moreover corrosive, and may cause deterioration of the equipment used. Moreover, the presence of an initiator is beneficial for controlling the molecular weights of the polymers.
Other methods for cationic copolymerization of ε-caprolactone have been proposed, which employ a sulfonic acid as catalyst instead of hydrochloric acid. The presence of an initiator is also necessary in these methods.
Such a method was notably described by Maigorzata Basko et al. in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 7071-7081 (2006). It consists of reacting ε-caprolactone and optionally L,L-lactide in the presence of isopropyl alcohol and trifluoromethanesulfonic (triflic) acid, in dichloromethane at 35° C. Caprolactone copolymers having a number-average molecular weight Mn in the range from 4780 to 5900 g·mol and a polydispersity index from 1.21 to 1.24 can be obtained in this way.
Polyamides, whose industrial interest requires no further demonstration, can be prepared by ring-opening polymerization. Among the techniques used, cationic polymerization based on acid, notably phosphoric, has been described extensively in the literature. Reference may be made in particular to application U.S. Pat. No. 3,846,357 of the present applicant.
For their part, carbonates, lactides and glycolides can be polymerized by various methods, mostly employing metal compounds.
However, it is not suggested that a method of the types described above can be used for preparing, in conditions that are generally mild and with rapid reaction kinetics, copolymers of lactones, lactams, carbonates, lactides or glycolides, oxazolines, epoxides, cyclosiloxanes in the presence of a substituted phosphorus-containing compound, optionally with an initiator bearing at least one hydroxyl or thiol function, the copolymers obtained possibly having a high number-average molecular weight Mn (optionally above 25 000 g/mol), a low polydispersity index (less than or equal to 1.5) and formation of stereocontrolled chains. A catalytic system employing substituted phosphorus-containing compounds of this type has the advantage of avoiding the use of metal compounds or highly acidic compounds. Moreover, it allows stereocontrolled chains to be obtained.